It was implied in TFA that he was a badass, I mean he stopped blaster bolts in mid air and telekinetically froze people and threw them through the air. Then suddenly he forgets he has these powers at the end of the movie and gets wasted by someone who has no training.
This is true. He was way more badass at the start of the movie than any previous SW villain (even Vader) and got progressively less badass (but in my opinion no less threatening) as the movie went on.
Rather than bad storytelling, I guess I saw his subsequent ‘weakening’ as a young, potentially super badass dude cracking under the weight of events and his emotions towards those events, and I found that quite fresh and interesting in a SW movie.
He wants to be a badass, and he has the power to be a badass, but when it comes to the crunch he’s not quite there yet. The intro set him up for what he could/should be, while the rest of the movie explored what he really is beneath the surface.
In a way, the OT actually did a similar thing with Vader, but spread it over 3 movies. In the first film, he was pure evil, a black and white comic book villain. In the second, events and revelations caused him to crack a little, and in the third he gave into the cracks. We don’t yet know how Kylo will deal with his own cracks, but killing his Father will either destabilise him further, or cement him into the dark side and turn him fully into the black and white villain some viewers (and indeed Kylo Ren himself) wanted him to be all along.